This invention relates to a wire bonder. More particularly, the present invention relates to a wire bonder which makes it possible to bond a thin wire of an easily oxidizable metal such as an aluminum wire, an aluminum alloy wire or the like by thermo-compression bonding.
A wire bonding step is one of the fabrication steps of a semiconductor device and is practised in order to electrically connect an electrode pad of an element pellet to a lead as an external conductor by use of a wire. At present, thermo-compression bonding using a gold wire (Au wire) and ultrasonic wire bonding using an aluminum wire (Al wire) have been used widely. Thermo-compression bonding of the Au wire is referred to as "nail head bonding". According to this technique, the tip of the Au wire which consists of an oxidation-resistant metal is heat-fused by a hydrogen torch or discharge arc to form a gold ball and this gold ball is compression-bonded while being pushed to a bonding portion using a wire connecting tool such as a capillary. This technique has the advantages that strong bond can be formed and the directivity of the wire bonding does not exist. The technique has the disadvantages that a so-called "purple plaque phenomenon" is formed (purple AuAl.sub.2 compound is produced when an Au-Al system is heated to 300.degree. C. or above) to reduce the bonding strength when the bonding portion is made of an aluminum material such as an electrode pad. Further gold itself is a precious metal which is subject to change in market price.
On the other hand, ultrasonic bonding using the Al wire fixes the Al wire by ultrasonic oscillation. This technique has the advantage that it can be practised economically because the aluminum wire is inexpensive. However, the construction of a wire bonder is complicated because wire bonding is effected by ultrasonic oscillation and because the directivity of wire bonding exists. Bonding speed is lower than the thermo-compression bonding.
Accordingly, in order to make the most of the advantages of both thermo-compression bonding and ultrasonic bonding techniques, a thermo-compression bonding technique using aluminum has been attempted in recent years. The advantages produced by this technique are that the bonding wire material, that is, aluminum, is inexpensive, no directivity exists in wire bonding, the wire bonder is simple in construction and the bonding speed is high. This technique is the so-called nail head bonding technique which forms a ball at the tip of the Al wire. If the conventional wire bonder using the Au wire and the hydrogen torch or discharge arc is applied to this technique, however, the ball can not be formed satisfactorily at the tip of the Al wire because aluminum is an easily oxidizable metal and the resulting oxide of aluminum exerts adverse influences. Thus, it has been found that wire bonding can not be carried out with high reliability in accordance with this technique.
An example of the conventional wire bonder of the thermo-compressing bonding system which uses the Al wire and forms a ball at the tip of the Al wire by discharge arc is disclosed in Japanese Patent Application No. 54834/1976 (Japanese Patent Laid-Open No. 147174/1976). As shown in FIG. 1, this wire bonder applies a potential across the Al wire 1 and a discharge electrode 2 from a high voltage source 3 while a negative potential is being applied to the Al wire to cause discharge arc between them while the tip of the Al wire is kept in the atmosphere of an inert gas such as argon (Ar) which causes the formation of the ball at the tip of the Al wire. However, the sphericity of the resulting ball is low and a constriction is formed at the neck of the Al wire immediately above the ball. It has been found that if wire bonding is performed under such conditions, the sphericity of the ball that has been compression-bonded is low, bonding to the correct position becomes difficult and the wire is likely to be broken from the constriction, reducing the reliability of bonding.
As a result of studies and analyses, the reasons why the sphericity of the ball of the Al wire is low and why the constriction occurs are that the aluminum oxide, i.e., alumina (Al.sub.2 O.sub.3), formed on the surface of the Al wire prevents the fusing of the Al wire and the formation of the ball and that discharge arc expands in the proximity of the tip of the Al wire, thereby causing energy dispersion.